docs(api): document the KOReader resolve endpoint

Add koreader/resolve_book.md for GET /api/sync/koreader/resolve, list
the endpoint in the API reference, and describe the resolve-then-pull-
then-push linking flow in the KOReader protocol page — including why a
device pushing to bootstrap its identity creates progress conflicts for
books already mid-read from other sources.

docs/developer/api/api-reference.md

@@ -237,6 +237,7 @@ See [OPDS Feeds](opds/)
 See [KOReader Sync](koreader/) and [Sync Protocol](sync/koreader-protocol.md)
 
 - POST /api/sync/koreader/progress - Sync reading progress
+- GET /api/sync/koreader/resolve?sha256={hash} - Resolve a book UUID by file SHA-256
 - GET /api/sync/koreader/metadata/:uuid - Get book metadata
 - GET /api/sync/koreader/library - Get device library
 - POST /api/sync/koreader/bookmarks - Sync bookmarks